The Moving Picture Experts Group (MPEG) has the concept of a Video Buffer Verifier, or VBV, which is a hypothetical decoder conceptually connected to the output of an encoder. When it is time for the VBV to decode a picture, it instantaneously removes it from its buffer, the VBV buffer. This VBV buffer must never overflow or underflow in normal operation. If it overflows, data is lost. If it underflows, video data is not present when needed to decode a picture.
A Bit-Rate Controller (BRC) function of a corresponding encoder is responsible for ensuring integrity of the VBV buffer. The BRC cannot do this directly, since it lacks direct access to the VBV buffer; instead it monitors the fullness of mBuff, its model of the VBV buffer.
For this reason, the overall encoding system is responsible for ensuring that mBuff correctly represents the VBV buffer. Any number of factors can cause these buffers to differ, and such differences can accumulate, causing the VBV buffer to overflow or underflow.
A specific problem relates to possible buffer overflow during processing of film to be played on television. When employing certain encoders, it is normal to complete encoding a picture (“pic”) early. In the film processing mode, however, the encoder may complete encoding of the picture a full field time early. When a picture is encoded early, the encoder may begin encoding the next picture early. When such encoding begins early, bits are put into the encoder buffer early, which creates a risk of writing them to a remote decoder prematurely. Even when encoding begins on time, bits may be written prematurely. This may cause the VBV buffer to overflow.